Fluxes of agricultural nitrogen and metolachlor metabolites are highly correlated in a first order stream in Maryland, USA

Nitrogen pollution in watersheds containing significant
cropland area is generally problematic. Conservation practices intended
to reduce nitrate-N (NO3--N) export from watersheds are being implemented
by many regions without necessary tools to assess effectiveness of these
abatement tools. A commonly used herbicide metolachlor degrades in the
vadose zone of croplands to form two metabolites (metolachlor ethane
sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA)) which are both
highly soluble in soils. Study of metabolite fates in a first order
watershed provided evidence that transport of these metabolites to stream
water is highly correlated to transport of the agricultural NO3--N that
also forms in the cropland vadose zone. Linear models describing the
relationships of stream flux of MESA and MOXA to NO3--N flux generated
goodness of fit values of 0.93 and 0.82 respectively. These findings
support a conclusion that both MESA and MOXA act as excellent transport
analogs of NO3- and become strongly correlated to agricultural NO3--N
leaching from the cropland vadose zone. Moreover, their use as
conservative tracers in agricultural watersheds can provide valuable
information concerning movement and fate of agricultural nitrogen at
watershed scales of observation.